1. Field of the Invention
This invention relates to an apparatus for controlling the position of a light beam, especially adapted for use in an optical information reading apparatus.
2. Description of the Prior Art
In the case where information stored in the surface of a recording medium having the shape of a disc (hereafter referred to simply as a disc) is read out by irradiating the disc surface with laser light etc., there is a need for both a focusing control for exactly focusing the light on the surface of the disc and a tracking control for causing the spot of the light to follow up the information track formed in the disc surface and moving constantly.
There have hitherto been proposed a variety of automatic focusing and tracking control methods and one example is the wobbling method. According to this method, a light beam which is subjected to a small oscillation having a constant frequency, is projected onto the surface of a disc, that part of the light which is reflected from or has passed through the disc is received by a light detector, and the positional deviation of the light beam is detected by measuring the amplitude of the output of the light detector, whereby the position of the light beam is controlled by operating the control system in such a manner that the positional deviation becomes zero.
First, the wobbling method as described above will be explained with the aid of the attached drawings. FIG. 1 schematically shows a conventional automatic focus control apparatus using the wobbling method. The laser light emitted from a laser light source 131 passes through a first lens 132 and a semi-transparent mirror 133, is reflected by a reflecting mirror 134 and converged by an objective lens 142, and is focused on the surface of a video disc 136 which is a moving object. The focused light is then reflected by the surface of the video disc 136 and sent back traveling the path along which it has reached the video disc. The light traveling back is again reflected by the semi-transparent mirror 133, passes through a pin hole in a diaphragm 137, and is received by a light detector 138. The diaphragm 137 may be omitted if the light detector 138 has a very small light receiving area. The information stored optically in the video disc 136 can thus be obtained as the output of the light detector 138, which is available at an information source terminal 139.
FIG. 2 shows the output characteristic of the light detector 138. In FIG. 2, the abscissae x represent the distance between the objective lens and the disc surface and the ordinates E represent the output of the light detector.
In the apparatus shown in FIG. 1, the diaphragm 137 is so located that the output of the light detector 138 becomes maximum when the light emitted from the objective lens 142 is focused exactly on the surface of the video disc 136. Let x.sub.o be the distance between the lens 142 and the surface of the video disc 136, assumed when the output E of the detector 138 is maximum. Then, as shown in FIG. 2, if x=x.sub.o, E equals its maximum value E.sub.o. This means that the focus of the objective lens 142 lies exactly on the surface of the video disc 136. If x&gt;x.sub.o, the focal point of the lens 142 is found to lie before the disc surface. The output of the light detector 138 varies almost symmetrically with respect to x.sub.o, as shown in FIG. 2. Accordingly, if the objective lens is oscillated in a direction parallel to the optical axis with the focal point of the lens located before the disc surface, that is, with the lens-to-surface distance x.sub.A greater than x.sub.o, then the output of the light detector oscillates out of phase by 180.degree. from the oscillation of the objective lens. On the other hand, if the lens is oscillated with x.sub.A kept smaller than x.sub.o, the oscillation of the output of the light detector is in phase with that of the objective lens. The excursion of the output E of the light detector increases with the increase in the absolute value of the difference between x.sub.A and x.sub.o.
In view of these facts, use is made of a means for synchronously rectifying the output of the light detector by multiplying the output by values varying in accordance with the mode of the oscillation of the objective lens. This means will be explained with the aid of FIG. 1.
First, an oscillator 140 generates a sinusoidal output, which is amplified by a current amplifier 148 and then supplied to a voice coil 141 to subject the objective lens 142 to a small oscillation in a direction parallel to the optical axis. As a result of this oscillation, the output of the light detector 138 undulates as seen in FIG. 2.
Since the output of the light detector 138 includes a signal having the same frequency as the frequency of the sinusoidal output plus other components, i.e. the video signal from the video disc 136 etc., superposed upon it, a band-pass filter 144 is provided to allow only the signal close in frequency to the sinusoidal output of the oscillator 140 to pass therethrough. Although the objective lens 142 is subjected to oscillation by the sinusoidal output of the oscillator 140, its oscillation lags in phase to a certain extent behind the sinusoidal signal. Therefore, after having been adjusted in phase to the oscillation of the lens 142 by a phase adjusting device 143, the sinusoidal output is supplied to a multiplier 145 to make a product of it and the output of the light detector 138.
The output of the light detector 138, near the peak of the characteristic curve shown in FIG. 2, can be approximated by a quadratic expression. Accordingly, if the objective lens is oscillated as expressed by b.multidot.sin .omega.t with the deviation of the focal point equal to (x.sub.A -x.sub.o), the output of the light detector 138 is given by the expression: EQU E=E.sub.o -a(x.sub.A +bsin .omega.t-x.sub.o).sup.2.
When the output E is multiplied by c sin .omega.t which is the sinusoidal signal representing the oscillation of the objective lens, the product e is as follows. ##EQU1##
In this case, the focus adjustment is performed through the control of the distance between the objective lens and the video disc by the voice coil 141 in accordance with the deviation of the focal point. The angular frequency .omega. should be chosen to be sufficiently higher than the angular frequency of the change in the deviation (x.sub.A -x.sub.o). When the signal representing the above product e is passed through a low-pass filter 146, the terms including sin .omega.t, cos 2.omega.t and sin 3.omega.t are eliminated and it follows that EQU e.perspectiveto.-abc(x.sub.A -x.sub.o).
That is, the output of the low-pass filter 146 gives 5-(x.sub.A -x.sub.o) which carries the information about the direction and the amount of deviation of the focal point.
In response to the output of the low-pass filter 146, a control circuit 147 delivers a control signal, which, after having been amplified by a current amplifier 148, is applied to the voice coil 141 so as to control the position of the objective lens in such a manner that (x.sub.A -x.sub.o) equals zero, that is, the focal point of the lens always lies on the surface of the video disc.
As described above, according to the wobbling method, the light beam emitted from a light source is slightly oscillated at a constant frequency (hereafter referred to as a wobbling frequency) in the direction parallel to the optical axis and directed upon the surface of a disc, that part of the light which is reflected from the surface of the disc or which has passed through the disc is received by a light detector, the direction and amount of the deviation of the focal point is measured by detecting the amplitude of the output of the light detector, whereby the focus control is effected. In this case, that component of the output of the light detector which has the wobbling frequency is exclusively extracted as the signal representing the deviation of the focal point. However, several components other than the wobbling frequency component, e.g. the signal representing the information stored on the disc, are superposed on the output of the light detector and therefore it is an essential problem for attaining stability in the control of the focal point to effectively detect only the wobbling frequency component from the detector output. In general, the wobbling frequency (e.g. about several tens of KHz) is about one order of magnitude lower than the frequencies in the frequency range (e.g. about several hundreds of KHz to 10 MHz) of the information signal and a band-pass filter has hitherto been used to extract only the signal having a frequency near the wobbling frequency. However, the band-pass filter has a poor efficiency for the separation of the desired signal. Accordingly, in order for the wobbling frequency component to serve sufficiently as the signal representing the focal deviation, it is necessary to increase the amplitude of the wobbling signal. If the amplitude of the wobbling signal becomes large, however, the information signal is modulated by the wobbling signal even in the absence of focal deviation. Consequently, the displacement caused by the wobbling cannot be neglected so that the stable control of the focal point becomes impossible.
In the preceding explanation, the control of focal point is exclusively explained, but a large wobbling signal will cause a similar adverse effect on the tracking control.